326 Bulletin American Museum of Natural History. \\o\. XX\'II, 



dorso-lumbars (D 12, L 7), and four endoturbinals with five scrolls. These 

 are, it is true, mostly primitive mammalian characters but they are not found 

 in this combination elsewhere and taken in connection with other characters, 

 are very significant. A comparison of the crania of Lepiis and Ochotona, 

 representing the Duplicentata, with those of Caprornys, and Pedetes, repre- 

 senting the Simplicidentata, reveals an ordinal agreement in the arrangement 

 of the cranial foramina, and in the characters of the petrosal, bulla, mastoid, 

 interparietal, condyles, palate, jugal, orbital and temporal fossa; (see below 

 p. 329). 



Origin of the Rodentia. 



Assuming then that the Duplicidentata and Simplicidentata are correctly 

 associated in the same order, ivhich of the superfamilij or subordinal groups 

 may he regarded as the most primitive, and in what respects does it approach 

 toward other orders f 



The known Lagomorpha (Duplicidentata), as stated above, have retained 

 very primitive characters in the reproductive system (Weber, 1904, pp. 485- 

 486) but the skull and skeleton have become highly specialized. 



The Hystricomorph series seems to be the least promising group in which 

 to look for primitive mammalian characters, in view of the presence of such 

 highly specialized characters as the greatly expanded antorbital fenestra, 

 the narrow palate, complexly folded hypsodont cheek teeth, and perfected 

 rasp-like action of the jaw in mastication. The known Tertiary Hystrico- 

 morphs exliibit the same characters. 



The Anomaluroidea (Pedetidje, Anoraaluridoe) mingle certain characters 

 of the Sciuromorphs, Myomorphs and Hystricomorphs with others of their 

 own but they do not appear to assist in the problem in hand. The Myo- 

 morpha are all obviously more specialized than the more primitive Sciuro- 

 morpha in the characters of the skull. 



Primitive characters of the Eocene genus Paraviys. — The Sciuromorpha 

 (including in this group both the typical forms and the Aplodontiidie, Geomy- 

 idse, Heteromyidae, etc.) embrace many forms with hypsodont complex 

 cheek teeth and highly specialized skulls; but among the Sciuridfe occur 

 also some relatively primitive forms with bunodont cheek teeth; and among 

 the presumable ancestors of this division we find in the Lower to Upper 

 Eocene genus Paramys, of the family Ischyromyidse, the most primitive 

 of all known Rodents. As figured by Cope (1884, pi. xxiva, figs. 1-14, 

 " Plesiarctomys" delicatissimus) and by Scott and Osborn (1889, pi. xi, 

 Paramys sciuroides), this genus differs from later Sciuromorphs notably in 

 the following primitive characters: the postorbital processes are slight or 



